In recent years, capacitors used in electronics have been required to reduce impedance (equivalent series resistance) in a high-frequency region with the digitization of the electronics. The capacitors have been further required more strictly to have long-term reliability with reduction in size and thickness and the diversified use environments of these electronics. To cope with these requirements, so-called functional capacitors (hereinafter, referred to as capacitors) containing an oxide film of a valve metal such as aluminum, tantalum or niobium as a dielectric have heretofore been used. The general structure of thus capacitor has, as shown in Patent Literature 1, an anode made of a porous body of a valve metal, a dielectric layer formed by oxidizing the surface of the anode, a conductive solid electrolyte layer, and a cathode having a laminate of a carbon layer, a silver layer and the like. A conductive film containing a pi-conjugated conductive polymer may be used as the solid electrolyte layer.
As for a technique using the conductive film containing such a pi-conjugated conductive polymer, a capacitor comprising a solid electrolyte of the configuration described above has been proposed, wherein the solid electrolyte consists of a composition having a pi-conjugated conductive polymer supplemented with a nitrogen-containing aromatic cyclic compound as an essential component (see Patent Literature 2). A feature of the composition constituting this solid electrolyte is that: the composition contributes to reduction in the equivalent series resistance (hereinafter, referred to as ESR) of the capacitor; and the capacitor can be produced by a convenient process of impregnation with a composition containing a pi-conjugated conductive polymer and drying.
Patent Literature 3 discloses an approach of coating an inside conductive polymer film formed on an anode oxide film of a valve-acting metal (e.g., tantalum, niobium or aluminum) with second polymer polymerization solution C to form an outside conductive polymer film with the conductive polymer polymerization solution, wherein the second polymer polymerization solution C is prepared as follows: a polymer consisting of a repeating structural unit of 3,4-ethylenedioxythiophene, and polystyrenesulfonic acid or a salt thereof are mixed into an aqueous solution, and stirred, followed by the addition of an oxidizing agent for chemical oxidative polymerization; and to the resulting first polymer polymerization solution A, mixed aqueous solution B of a non-aqueous solvent and pure water containing naphthalenesulfonic acid or the like dissolved therein is added.
Likewise, Patent Literature 4 discloses an approach of preparing a solid electrolytic capacitor by establishing a conductive polymer layer on an anode oxide film having a precoat layer and an inside conductive polymer layer formed in order on the surface by coating or impregnation with a water dispersion comprising PEDOT and PSSA containing naphthalenesulfonic acids, high-molecular-weight PSSA, boric acid, mannitol, glycols and the like, and drying and solidifying the resultant by heating. The sulfo group of PSSA with which PEDOT is doped imparts water dispersibility to the water dispersion, but has the function of decreasing the pH of the solution to a strongly acidic region. Therefore, the anode itself is damaged by strong acidity, disadvantageously resulting in elevation in ESR or decrease in capacitance. To solve this disadvantage, a method for increasing pH by the addition of an amine compound has also been proposed (see, for example, Patent Literature 5).